src: only set JSStreamWrap write req after write()

Otherwise `this[kCurrentWriteRequest]` is set to a value even
if one of the `write` calls throws.

This is needed in order not to break tests in a later commit.

PR-URL: #18676
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Anatoli Papirovski <apapirovski@mac.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

lib/internal/wrap_js_stream.js

@@ -137,7 +137,6 @@ class JSStreamWrap extends Socket {
   doWrite(req, bufs) {
     assert.strictEqual(this[kCurrentWriteRequest], null);
     assert.strictEqual(this[kCurrentShutdownRequest], null);
-    this[kCurrentWriteRequest] = req;
 
     const handle = this._handle;
     const self = this;
@@ -149,6 +148,9 @@ class JSStreamWrap extends Socket {
       this.stream.write(bufs[i], done);
     this.stream.uncork();
 
+    // Only set the request here, because the `write()` calls could throw.
+    this[kCurrentWriteRequest] = req;
+
     function done(err) {
       if (!err && --pending !== 0)
         return;